Interleukin 10 (IL-10) is a cytokine which is intimately involved in the regulation of immune and inflammatory responses. Several studies have documented its essential role in preventing prolonged and exaggerated immune responses to antigens and irritants. The safety, tolerance, immunologic effects and clinical activity of multiple doses of subcutaneous recombinant human IL-10 are evaluated in 29 patients with psoriatic arthritis. In this randomized, double-blind, placebo-controlled, escalating, multiple dose trial, patients receive a 28 day course of daily subcutaneous IL-10 (1.0, 5.0, 10.0 or 20.0 mg/kg) or placebo. At the end of the study, patients receiving placebo have the option to receive IL-10 (20 mg/kg). Disease activity (both skin and joints) and toxicity are being monitored throughout the study. Research studies include functional assays of peripheral blood and synovial fluid mononuclear cells; cytokine gene expression (blood, synovium, skin); and activation of transcription factors specific for IL-10. To date, a total of 16 patients have been enrolled into the study.